The present invention relates to NOx emission reduction in power plants without loss of thermal efficiency, and in particular, to the utilization of flameless oxidation to achieve NOx emissions reduction in adiabatic combustors such as those used in gas turbine engines.
Awareness and sensitivity to environmental issues have been increasing around the world, and in their wake, environmental legislation has dictated increasingly strict standards for stationary, propulsive and vehicular power-plant emissions, including the emission of NOx gases. NOx gases are formed mainly at high temperatures and contribute to smog and acid rain at low levels of the atmosphere, and to stratospheric ozone depletion. Carbon dioxide (CO2), another emitted pollutant, is directly linked to the greenhouse effect. Because CO2 is a natural product of efficient hydrocarbon combustion, there is no way of avoiding CO2 production in a combustor using conventional fuels. Hence, a reduction in CO2 emissions from the various kinds of power plants operating with fossil fuels can be obtained only by improvements in the overall thermal efficiency of the system.
Although increased combustor temperatures and pressure ratios improve gas turbine power rating and efficiency, these conditions in conventional combustors tend to promote NOx formation, such that there is a natural conflict between energy savings and combustion performance on one hand and reduction of pollutant emission on the other hand. Thus, in order to improve gas turbine efficiency, it has been necessary to develop low-NOx combustion systems. These systems can be divided into two groups of methodologies, one based on post-treatment of flue gases to reduce NOx levels, and the other based on modification of the internal combustion process. This category can be further divided into two main groups; xe2x80x9cdryxe2x80x9d techniques in which no additives to the fuel and air supply are applied, and xe2x80x9cnon-dryxe2x80x9d techniques using steam or water injection for flame cooling. The present invention is concerned with the dry techniques of the second category.
The dry techniques include the following main methods:
1. Staged Combustion
Both in the classic technology of fuel staging, which has actually been implemented in commercial service, and in variable geometry (air-staging) technology, the designs introduce additional mechanical complexity and control problems, e.g., moving parts in the case of variable geometry and multi-fuel injection system in the case of fuel-staging. In addition, the pollutant reduction potential is only moderate. In the pilot diffusion flame of a staged combustor, a large amount of NOx is still produced. Moreover, radially-staged combustors of current design have pattern factors at the turbine inlet which are far from uniform, such that the potential reduction in NOx emissions is limited.
2. Lean Pre-vaporized Premixed Combustion (LPP)
LPP technologies are based on the combustion of a Lean Pre-vaporized and Pre-mixed mixture to reduce the maximum flame temperature. LPP requires operation of a pre-mixer, which can be damaged by flashback or by auto-ignition of the air-fuel mixture. In addition, leakage of fuel or gases from the pre-mixer into the hot section of the combustor may result in severe failures and even explosion of the engine casing. These safety problems appear to be even more pronounced when using liquid fuels, because of the longer time required for complete pre-evaporation. In addition, LPP can not be used at high air inlet temperatures because under such conditions, the mixture is even more susceptible to early auto-ignition. Moreover, it is known that pre-mixed combustion can lead to combustion instabilities that shorten combustor lifetime. In addition, in order to be fully effective under a wide range of operating conditions and to avoid blow-off at idle or partial loading, the LPP system must be coupled to a variable geometry system.
3. Rich Quench Lean (RQL) Combustion
The Rich-Quench-Lean (RQL) combustion methods are based on a rich combustion phase in a reducing combustion environment followed by a lean combustion to complete the burnout. The main advantage of the rich zone is that it allows reduction of NOx emissions from Fuel Bound Nitrogen and avoids thermal-NO formation by remaining far in excess of the stoichiometric fuel to air ratio. However, RQL requires physical separation of the combustor into two chambers, rich and lean, as well as an intermediate transition passage known as the quenching zone. RQL technologies also require a special form of cooling for the rich combustion zone. In addition, the primary zone generates a large amount of soot, which radiates heat to the walls, thereby aggravating the cooling problem. The RQL method is limited by the practical difficulty in realizing an effective and uniform quenching between the rich zone and the lean zone. This is due to the fact that in the quenching zone, the stoichiometric ratio is reduced below unity. The requisite degree of complexity to achieve the careful balance between the rich-burn zone and the lean-burn zone over the full range of operation of a gas turbine combustor is subtantial, and consequently, such a balance has yet to be fully realized to date.
4. Catalytic Combustion
Catalytic combustion allows fuel oxidation to take place at temperatures well below the lean flammability limit of the fuel/air pre-mixture. Catalytic combustion can decrease the NOx emissions by several orders of magnitude. However, the concept is not easily applicable to non-stationary power-plants and has several drawbacks: catalytic combustion requires relatively high inlet temperatures (depending on the catalyst), and therefore requires a control system for inlet conditions. Because of the premixing, there is also risk of auto-ignition of the premixed mixture before the catalytic bed and consequent flashback, which can lead to catastrophic failures. The catalytic bed increases engine weight and pressure losses. In addition, the catalytic beds of today still reduce drastically the reliability and lifetime of the combustor. Therefore, catalytic combustion is not yet a viable technology, particularly for aircraft applications.
5. Exhaust Gas Recirculation (EGR)
Exhaust gas recycling, whether it is internal or external, is an effective method to reduce flame temperature and, thereby, nitrogen oxide emissions. Unfortunately, the efficiency of this method is limited by the maximal available quantity of recirculated exhaust gas since flame instabilities and ultimately blowout can occur if the burner is operated at overly-high recirculation rates. External recirculation is feasible only if the temperature of the recirculated exhaust gas is relatively low, typically about 850xc2x0 K, as is the case for industrial furnaces. Recirculation at higher temperatures is impractical, mainly due to external piping limitations and thermodynamic losses. In addition, external recirculation is viable specifically in furnace-type applications because such applications are essentially free of geometrical constraints and weight considerations.
The deficiencies in these alternative combustion technologies are particularly manifest in renewable energy applications, such as the combustion of synthesis gas produced from the gasification or pyrolysis of biomass, including municipal waste. Although renewable energy utilization has become an integral part of the energy policies of the European community and the United States, the efficient exploitation of synthetic gas is not widespread because of various technological difficulties. These technological difficulties mainly arise from the LHV (Low calorific Heat Value) of such fuels, which requires operation at super-adiabatic temperatures. In addition, the relatively-high laminar flame speed makes premixing systems using synthesis gas susceptible to combustion instabilities, including auto ignition and flash-back, both of which have an extremely deleterious impact on safety and on NOx emissions.
Consequently, the utilization of synthesis gas has been largely limited, until now, to atmospheric pressure combustion that results in low-efficiency cycles. Such restrictions preclude the use of synthesis gas in electric power plants using the highly efficient, combined Rankin (low-pressure combustion) and Brayton (high-pressure combustion) thermodynamic cycle.
Flameless Oxidation
It has been found recently that under special conditions, it is possible to achieve a stable form of combustion at high exhaust gas recirculation rates. If the temperature of the recycled exhaust gas exceeds the auto-ignition temperature of the fuel, the fuel is ignited automatically and continuous combustion is sustained. In this flameless oxidation mode, in contrast to the classic diffusion flame, temperature peaks can be avoided even at high air preheat temperatures. This combustion mode is characterized by moderate and distributed temperature rise, small temperature and concentration gradients, low radiation emission and low noise levels. Under these conditions, the thermal-NOx formation can be largely suppressed. Recent experimental studies have shown that NOx emissions decrease drastically with the decrease of oxygen concentration in a nitrogen-diluted air stream, especially at high temperatures. This effect could be primarily attributed to reduction of flame temperature, and reduction of O and OH radicals in the flame.
Although many fundamental issues regarding this combustion method still require further investigation, field results conclusively demonstrate the effectiveness of flameless oxidation in reducing NOx emission levels, even at high operating temperatures. Until now, however flameless oxidation combustion has been applied only in industrial furnaces at atmospheric pressure, using high-momentum jets to locally recirculate a portion of the combustion products.
There are several important reasons why adiabatic external EGR methods (without deliberate heat extraction) and current flameless oxidation systems are not practical for combustion in gas turbines. In sharp contrast to industrial furnaces, gas-fueled and liquid-fueled gas turbines are designed to provide power by adiabatic expansion of the combustion gases from high pressure (typical values are from 4 bar to about 40 bar) to about atmospheric pressure, hence combustion occurs at significantly elevated pressure. In addition, the turbine entry temperature, typically 1100-1700xc2x0 K, is significantly higher than the exhaust temperature of non-adiabatic industrial furnaces. External recirculation of high-pressure exhaust gases in the above temperature range is thermodynamically inefficient and practically impossible due to a large volume and weight associated therewith. In addition, special, expensive construction materials would be required.
Moreover, geometrical constraints do not allow the implementation of external recirculation in many applications. Additional considerations, including weight and aerodynamics, make external recirculation particularly impractical for use in conjunction with gas turbines and aero-engines.
Recirculation by means of an ejector or high-momentum jet is possible for some applications, however, the ultra-high velocity of the discharge gas impairs the mixing of the gas streams. The poor mixing deleteriously affects the reduction of NOx emissions and leads to hot spots in the combustor. This can be partially overcome by the application of numerous discrete jets, but this is a rather cumbersome and expensive engineering solution. It must be emphasized that in ejectors, the ratio between motive gas and suction gas is delicate and, more significantly, the ejector performance constrains the ratio to a value that is far from optimal with respect to NOx emissions and with respect to combustion efficiency.
There is therefore a need for low-cost, safe and reliable NOx reduction methods that are applicable to gas turbines, and more particularly, to aero-engines. There is also a need for a combustion technology that improves the combustion efficiency and reduces carbon dioxide emissions. Finally, there is a need for a combustion technology that enables safe and efficient utilization of synthesis gas and other low-calorific renewable energy sources.
It is an object of the present invention to provide a combustor for industrial gas turbines and aero-engines that produces low pollutant emission levels.
It is an object of the present invention to provide an improved combustor for gas turbines, ground and marine vehicular applications and aero gas turbines and jet engines that produces NOx emissions at levels considerably below the acceptable level according to the most stringent environmental regulations, while allowing operation at high temperature to improve thermal efficiency.
It is a specific object of the present invention to provide an improved combustor for gas turbines and aero-engines in which NOx formation is controlled, even at high inlet air temperature conditions, such that high-efficiency combine cycles like the Rankin-Brayton thermodynamic cycle can be applied.
It is an object of the present invention to provide a combustor for gas turbines and aero-engines that can safely and efficiently utilize gaseous and liquid fuels including synthetic gas and other low-heat value fuels.
It is an object of the present invention to provide a combustor for industrial gas turbines and aero-engines that is robust, simple to operate, and inexpensive relative to technologies of the prior art.
Finally, it is an object of the present invention to provide a combustor for industrial gas turbines and aero-engines that can be retrofitted in existing systems.
The present invention is an improved combustor design principle for industrial gas turbine engines, aero-engines, jet engines and the like, that achieves stable, flameless oxidation by internal recirculation of burned products, thereby improving combustion efficiency and reducing NOx emissions. The internal recirculation is achieved by modifying the shapes and positions of the primary and additional air inlets and the shape and positions of the fuel injector(s) to induce the formation of a large recirculation zone in which direct combustion of the fuel in the fresh air flow is avoided. The combustion of fuel in the hot vitiated air avoids temperature peaks and evens the temperature distribution and therefore reduces the production of NOx gases without compromising flame stability.
According to the teachings of the present invention there is provided a combustor for energy-production systems including: a) a combustion chamber producing pressurized combustion gases and having a primary zone containing a substantially vitiated-air zone; b) a primary air inlet providing air to the primary zone, and c) a fuel injector for injecting fuel, located in the primary zone of the combustion chamber, wherein the fuel injector introduces the fuel into the substantially vitiated-air zone within the primary zone of the combustion chamber to achieve flameless oxidation.
According to another aspect of the present invention there is provided a combustor for energy-production systems including a) a combustion chamber producing pressurized combustion gases and having a primary zone containing a substantially vitiated-air zone; b) a primary air inlet providing air to the primary zone, and c) a fuel injector for injecting fuel, located in the primary zone, wherein a portion of the pressurized combustion gases undergoes internal recirculation in the combustion chamber, and wherein the fuel is introduced by the fuel injector into the substantially vitiated-air zone within the combustion chamber to achieve flameless oxidation.
According to another aspect of the present invention there is provided a combustor for gas turbines including: a) a combustion chamber, encompassed by a wall, producing pressurized combustion gases and having a primary zone containing a substantially vitiated-air zone; b) a primary air inlet providing air to the primary zone; c) a fuel injector for injecting fuel, located in the primary zone, wherein the fuel injector introduces the fuel into the substantially vitiated-air zone within the primary zone of the combustion chamber to achieve flameless oxidation.
According to further features in preferred embodiments of the invention described below, the internal recirculation is achieved by means of a vortex. According to still further features in preferred embodiments of the invention described below, the injected fuel has momentum that is used to augment and stabilize the circulation of the internal vortex.
According to still further features in preferred embodiments of the invention described below, the primary air inlet provides substantially all of the air introduced to the combustion chamber.
According to still further features in preferred embodiments of the invention described below, the combustor further includes at least one secondary inlet.
According to still further features in preferred embodiments of the invention described below, the fuel is a hydrocarbon fuel selected from the group consisting of gaseous fuel, liquid fuel, synthesis gas, and low calorific gas. According to still further features in preferred embodiments of the invention described below, the synthesis gas is produced from an energy source selected from the group consisting of coal, biomass and waste.
According to still further features in preferred embodiments of the invention described below, the combustion chamber wall has an internal surface having an average temperature below 1500xc2x0 K and a maximum temperature below 2000xc2x0 K.
According to further features in preferred embodiments of the invention described below, the pressurized combustion exhaust gases have a NOx level below 20 ppmv. According to still further features in preferred embodiments of the invention described below, the pressurized combustion gases have a NOx level below 10 ppmv.
According to still further features in preferred embodiments of the invention described below, the pressurized combustion gases are discharged from the combustion chamber at a temperature of at least 1600xc2x0 K and have a NOx level below 20 ppmv. According to still further features in preferred embodiments of the invention described below, the pressurized combustion gases are discharged from the combustion chamber at a temperature of at least 1800xc2x0 K and have a NOx level below 20 ppmv. According to still further features in preferred embodiments of the invention described below, the pressurized combustion gases are discharged from the combustion chamber at a temperature of at least 1600xc2x0 K and have a NOx level below 10 ppmv.
In yet another preferred embodiment, the positioning of the primary and secondary air inlets and fuel injection system are totally separated and oriented, as described in further detail below, such that the fuel is injected into substantially vitiated air.
The global flow parameters in the flameless oxidation combustor of the present invention are similar to those of conventional combustors, such that only minor changes in the ducts between the compressors and the combustor are necessary to implement the flameless oxidation technology in existing combustors.
Thus, in a preferred embodiment of the present invention, the combustor design is applied to existing, conventional combustors of gas turbines and the like (retrofits). Unlike other kinds of modifications, such as catalytic combustion and water/steam/ammonia injection, the combustion method of the present invention requires no auxiliary equipment and no external supply of additional fluids as in alternative processing methods like exhaust gas de-NOx and flame cooling.
The combustor design of the present invention with internal recirculation overcomes the problems associated with adiabatic combustors of the prior art, and enables flameless oxidation to be applied to adiabatic and high-pressure applications.
For the purposes of this specification and the accompanying claims, the term xe2x80x9chigh-pressurexe2x80x9d with respect to gas turbines, combustion chambers, combustion gases, and the like refers to pressures exceeding 4 bar.
For the purposes of this specification and the accompanying claims, the term xe2x80x9cadiabaticxe2x80x9d with respect to gas turbines, combustion chambers, and the like, refers to such systems having walls through which there is no designed, deliberate heat extraction. It will be appreciated by those skilled in the art that while heat losses occur in all real systems, making the theoretical adiabatic condition unattainable, many practical systems are designed to approach the adiabatic condition, and any heat interactions with the environment are simply heat losses. By sharp contrast, many practical systems, such as industrial furnaces, heat is deliberately extracted from the combustion gases via a wall or other heat-exchange surface.
For the purposes of this specification and the accompanying claims, the term xe2x80x9cgas turbinexe2x80x9d or xe2x80x9cgas turbinesxe2x80x9d includes a wide variety of gas turbines, including but not limited to xe2x80x9copenxe2x80x9d cycle gas turbines, with or without regeneration; combined Brayton/Rankin Cycle power generation systems, and aero-engines.
For the purposes of this specification and the accompanying claims, the term xe2x80x9cenergy-production systemsxe2x80x9d refers to a wide variety of both small and large energy-production systems, including electric power generation by xe2x80x9copenxe2x80x9d cycle gas turbines, with or without regeneration (heat exchangers); combined Brayton/Rankin Cycle power systems using steam and gas turbines for power and heat generation; aero-engines, and other applications in which the combustion products are pressurized and can undergo internal recirculation.
For the purposes of this specification and the accompanying claims, the term xe2x80x9cvitiated airxe2x80x9d refers to air containing an appreciable amount of combusted product gases, such that the oxygen available for combustion has been partially consumed. The amount of available oxygen in the vitiated air is less than 18%, preferably less than 16%, more preferably less than 14% and most preferably less than 12%.
For the purposes of this specification and the accompanying claims, the terms xe2x80x9cflameless oxidationxe2x80x9d and xe2x80x9cflameless combustionxe2x80x9d refer to a mode of combustion wherein the fuel comes into contact with vitiated air, and wherein the temperature of the recycled exhaust gas exceeds the auto-ignition temperature of the fuel, such that sustained and stable combustion is achieved.